Stabilizing circuit



May 2, 1939.

D. V. SINNINGER STABILIZING CIRCUIT Filed June 18, 1937 INVENTOR DWIGHT V. SIN/V/NGER M1; @26 ATTORNEY Patented May 2, 1939 UNITED STATES PATENT OFFICE STABILIZING omcmr poration of Illinois Application June 18, 1937, Serial No. 148,927

7 Claims.

This invention relates to improvements in high-frequency amplifying systems, and more particularly to vacuum-tube amplifiers of the type commonly employed in receivers for modulated high-frequency signals. Although applicable to various types of high-frequency amplifiers, the invention is especially useful in amplifiers intended for operation over a limited range of frequencies, as for example the intermediate-frequency amplifier of a radio receiver of the superheterodyne type. Such an amplifier is usually initially adjusted for optimum performance at a single fixed frequency, but is designed to pass F a relatively narrow band of frequencies lying on either side of its nominal resonant frequency.

In vacuum-tube amplifiers intended for operation at high frequencies, the electrical characteristics of the vacuum tubes themselves introduce several serious problems which must be solved in the design of a satisfactory high-gain amplifier. One of these difiiculties is the inherent grid-plate capacitance of the vacuum tube. This capacitance permits a certain amount of feedback Y from the plate circuitto the grid circuit of the vacuum tube, and, in a high-gain amplifier, this feedback may be sufiicient to produce instability and oscillation. Although the effective gridplate capacitance may be reduced substantially by the use of a screen-grid within the vacuum tube itself, in many cases it is sufiicient to introduce serious difiiculty in the design of a high-gain amplifier.

Another difficulty is encountered when a selfbiasing resistor is inserted between the cathode 35 of the vacuum tube and ground. It is common practice to shunt this resistor with a by-pass capacitor, and this capacitor acts as an impedance which is common to the grid and plate circuits of the vacuum tube with resultant instability 40 of operation. Even the use of a relatively large and expensive capacitor across the cathode resistor is not a positive cure for this source of instability.

An object of this invention is to provide simple 45 and inexpensive means for substantially eliminating the undesirable effects of vacuum-tube grid-plate capacitance in high-gain vacuum-tube amplifiers.

Another object of the invention is to provide 50 means for preventing feedback and instability due to the use of a common impedance in the cathode circuit of a vacuum-tube amplifier.

An additional object of the invention is to provide means for substantially increasing the stable amplification which may be realized in a highgain vacuum-tube amplifier without appreciably increasing its cost and complication.

Although the arrangements of the invention are applicable to various types of high-frequency vacuum-tube amplifiers, they are especially useful 6 in intermediate-frequency amplifiers which are designed to have a flat-topped, steep-sided selectivity characteristic, since feedback has a marked tendency to alter the symmetry of the amplifier characteristic and thus introduce serious distortion of the modulated high-frequency signal being amplified. It is a further object of the invention, therefore, to provide a high-frequency amplifier which is capable of being adjusted to have a substantially symmetrical selectivity characteristic of a desired form.

Although the arrangements of the invention are especially suitable for use in an amplifier employing one or more vacuum tubes of the type having a metal outer envelope or shell, it will 120 be understood that tubes of the type having a glass envelope may be employed by providing them with a close-fitting shield of conductive material. Whenever in this specification and the appended claims the term shell is used, there- 25 fore, it is intendedto refer either to the metal outer envelope of the tube itself or to a closefitting conductive shield surrounding the glass envelope of the tube.

The invention will be better understood by reference to the drawing, in which:

Fig. 1 is the schematic wiring diagram of a vacuum-tube amplifier employing the arrangements of the invention;

Fig. 2 is a modification of the system of Fig. 1; and

Fig. 3 is a second modification of the system of Fig. 1.

Referring to Fig. 1 of the drawing, tuned circuit I is in the grid or input circuit of vacuum tube 2. 40 The grid circuit is completed for high-frequency currents by means of capacitor 3, and a directcurrent bias voltage is applied to grid 4 through filter resistor 5 from source AVG. The high-frequency plate or output circuit of vacuum tube 2 extends from plate 6 through tuned circuit 1 and capacitor 8 to ground. Cathode 9 of vacuum tube 2 is grounded through resistor l0, which is shunted by capacitor H. Screen-grid I3 is bypassed to ground by capacitor 14, and suppressorgrid I5 is connected to cathode 9. Shell N5 of vacuum tube 2 is grounded through inductor I1.

If vacuum tube 2 is of the glass-envelope type,

it will be understood that a close-fitting conduc- 5-5 ISO tive shield is to be employed and connected as shown for shell I 6.

The capacitance between grid 4 and shell 16 is indicated by dotted capacitor l8, and the capacitance between plate 6 and shell I6 is represented by dotted capacitor 19. The capacitance between grid 4 and plate 6 is depicted by dotted capacitor 20. Reference to capacitors I8, l9 and 20, therefore, is to be taken as meaning the grid-shell, plate-shell, and grid-plate capaci tances, respectively, of vacuum tube 2, rather than separate and. distinct capacitively reactive circuit components.

In operation, regenerative feedback from the plate circuit to the grid circuit takes place through capacitance 20. If the amplification of vacuum tube 2 is high enough, this regeneration is sufficiently great to produce oscillation. In most cases, there is suificient regeneration to produce serious instability in high-gain amplifiers. In the arrangements of the present invention illustrated in Figs. 1 and 3, the regeneration is counteracted by regenerative feedback produced by making inductor l1 common to both the grid and the plate circuits.

Capacitance I8 and inductor H are effectively in series between grid 4 and ground, and thus some of the current in the input circuit flows through inductor l'i. Likewise, capacitance l9 and inductor H are effectively in series between plate 6 and ground, causing some of the platecircuit current to 'be present in inductor H. The phase relations between'these stray'input and output currents is such that there is developed across inductor l! a voltage which offsets the effect of the regenerative coupling between the input and output circuits due to capacitance 20. By properly choosing the value of inductor I 1 for any given type of vacuum tube to be operated at a given frequency, the effects of feedback due to grid-plate capacitance may be substan tially eliminated. Thus the operation of a highgain amplifier may be rendered stable and free from undesired regeneration.

Reference is now made to Fig. 2 of the drawing, in which like components are designated by like reference numerals. Shell iii of vacuum tube 2 is connected to the junction of capacitor 3 and resistor 5. The value of capacitor 3 is so chosen as to provide shell l6 with the proper potential to balance out the tendency of the gridplate capacitance 20 to produce feedback and resultant regeneration. The capacitive reactance of capacitor 3 is so small, however, that shell [6 is practically at ground potential and thus fully effective as a shield for vacuum tube 2. The arrangement of Fig. 2 has the advan tage of requiring no additional components and of being independent of the frequency at which the amplifier operates.

Capacitor 3 is common to the grid and plate circuits of vacuum tube 2, since the grid circuit is completed to ground through capacitor 3 and one path from plate 6 to ground includes plateshell capacitance l9 and capacitor 3. By properly choosing capacitor 3, the potential drop across it may be made to substantially cancel the undesired regenerative voltage which is impressed on grid 4 due to grid-plate capacitance 20. Because of the phase shift due to tuned circuit I, it is possible thus to employ a capacitor as the common circuit element, instead of an inductor as shown in Fig. 1.

' Fig. 3 of the drawing shows the system of Fig. 1 to which has been added means for preventing feedback and instability due to the use of a common impedance in the cathode circuit of the vacuum-tube amplifier. Like components bear like reference numerals. Resistor I0 is shunted by capacitors H and 12 in series, and the lowpotential terminal of tuned circuit 1 is connected tothe junction of capacitors II and I2. The capacitance between grid 4 of vacuum tube 2 and ground is indicated by dotted capacitor 2|,

and the capacitance between grid 4 and cathode I 9 of vacuum tube 2 is represented by dotted capacitor 22. The value of capacitance 2! depends both upon the type of vacuum tube employed and the arrangement of components associated with it, and the value of capacitance 22 is determined only by the type of tube.

In practice, the circuit components are so chosen that capacitance 2| is equal to capacitance 22;, and that capacitors I l and [2 are equal to each other. Under these conditions, none of the current in the input or grid circuit of vacuum tube 2 is permitted to flow through cathode resistor ill, so there can be no feedback due to cathode-circuit coupling between grid and plate circuits. Feedback and instability due to capacitance 20 is eliminated in the manner explained in connection with Fig. 1 of the drawing, so that the system is entirely free from either regeneration or degeneration. The arrangements of Fig.

3 are especially advantageous in high-gain amplifiers in which the cathode cannot be by-passed with a large capacitor .or grounded, and this embodiment'has the feature of requiring minimum additional components. Additionally, the means for preventing feedback due to the common impedance in the cathode circuit is substantially independent of the frequency at which the amplifier operates.

By way ofexample of one successful embodiment of the arrangements shown in Fig. 3 of the drawing, the following constants are given for a system designed to operate at a frequency of 460 kilocycles per second:

Vacuum tube 1 Type 6K7 Capacitor 8 0.1 microfarad Resistor 10 300 ohms Capacitor 11 0.02 'microfarad Capacitor 12 0.02 microfarad Capacitor 14 0.05 microfarad Inductor 1'7 12.0 microhenries It will be understood that these are approximate values and that the scope of the invention is in no way limited to embodiments employing the same or even closely similar values. A different operating frequency or the use of different types of vacuum tubes may require different values for some of the constants, and such different values are contemplated by the invention.

In some instances, cathode 9 of vacuum tube 2 may be grounded. In this event, the problem of common impedance in the cathode circuit does not arise, and complete stability is obtained as above disclosed in connection with Figs. 1 and 2. The value of inductor I1 is not critical, but in some instances it may be desirable to provide means for adjusting its inductance in order to secure a precise balance of the system.

Although the arrangements of the invention are especially suitable for use in radio receivers, and'particularly in the intermediate-frequency portion of superheterodyne radio receivers, it will be understood that they may be employed in radio transmitters and also in other types of high-frequency apparatus in which it is desired to secure a high degree of efliciency without sacrificing stability. Furthermore, although a pentode vacuum tube is shown in the figures of the drawing, the principles of the invention may be applied to vacuum tubes of any other type to realize a high degree of stable amplification.

Having thus described my invention, what I claim is:

l. A high-frequency amplifier including a vacuum tube having input and output electrodes in proximity and a conductive shell, an input circuit connected between said input electrode and ground, an output circuit connected between said output electrode and ground, and a reactance effectively connected between said shell and ground of such size that feedback from said output circuit to said input circuit due to the proximity of said input and output electrodes is substantially prevented.

2. A high-frequency amplifierincluding a vacuum tube having input and output electrodes in proximity and a conductive shell, an input circuit connected between said input electrode and ground, an output circuit connected between said output electrode and ground, and an inductive reactance effectively connected between said shell and ground of such size that feedback from said output circuit ,to said input circuit due to the proximity of said input and output electrodes is substantially prevented.

3. A high-frequency amplifier including a vacuum tube having input and output electrodes in proximity and a conductive shell, an input circuit connected between said input electrode and ground, an output circuit connected between said output electrode and ground, and a capacitive reactance effectively connected between said shell and ground of such size that feedback from said output circuit to said input circuit due to the proximity of said input and output electrodes is substantially prevented.

4. A high-frequency amplifier including a vacuum tube having input and output electrodes in proximity and a conductive shell, an input circuit connected between said input electrode and ground and including a first resonant circuit tuned to a predetermined frequency, an output circuit connected between said output electrode and ground and including a second resonant circuit tuned to said frequency, and a reactance effectively connected between said shell and ground of such size that feedback from said output circuit to said input circuit at said frequency due to the proximity of said input and output electrodes is substantially prevented.

5. A high-frequency amplifier including a vacuum tube having input and output electrodes in proximity and a conductive shell, an input circuit connected between said input electrode and ground and including a first resonant circuit tuned to a predetermined frequency, an output circuit connected between said output electrode and ground and including a second resonant circuit tuned to said frequency, and an inductive reactance effectively connected between said shell and ground of such size that feedback from said output circuit to said input circuit at said frequency due to the proximity of said input and output electrodes is substantially prevented.

6. A high-frequency amplifier including a vacuum tube having input and output electrodes in proximity and a conductive shell, an input circuit connected between said input electrode and ground and including a first resonant circuit tuned to a predetermined frequency, an output circuit connected between said output electrode and ground and including a second resonant circuit tuned to said frequency, and a capacitive reactance efiectively connected between said shell and ground of such size that feedback from said output circuit to said input circuit at said frequency due tothe proximity of said input and output electrodes is substantially prevented.

7. A high-frequency amplifier including a vacuum tube having input and output electrodes in proximity, a cathode and a conductive shell, an impedance including a resistor shunted by two capacitors in series connected between said cathode and ground, an input circuit connected between said input electrode and the junction of said capacitors, an output circuit connected between said output electrode and ground, and an inductive reactance connected between said shell and ground, said capacitors and said inductive reactance being of such sizes that instability due to the proximity of said input and output electrodes and degeneration due to said impedance are substantially prevented.

DWIGHT V. SINNINGER. 

